Protein trafficking Flashcards
(32 cards)
2 ways to transport from 1 membrane compartment to another
- Vesicular transport
- budding, movement through cytoskeletal -> fusion - Direct fusion
- hybrid organelle
The function of coats for transport vesicles:
- structural - form and deform
2. selective of which proteins come in and out
How are clathrin vesicles formed?
- Ligand binds to receptor
- receptor recognised by adaptor
- clathrin + adaptor recruit at plamsa membrane + start to bud
What type of structure is formed by clathrin and adaptors?
Big lattice like structure
Elaborate on the process of budding
Neck is pinched by GTPase -> vesicle -> as soon as coat forms its disassembled by Hsp70
-> clathrin and adaptors forms another vesicle
What are the functions of the two different SNARE proteins?
t-SNARE : fusion from target membrane
v-SNARE: fusion from vesicle membrane
Mechanism of fusion in endocytosis
- Uncoated - V-snare exposed
- Targeting recruits Rab proteins
- Rab binds to effector
- V-snare provides 1 alpha helix + t-SNARE provides 3 helixes
- 4 helical bundles fuse -> membrane fuses
Trans-snare pairing
Interaction across 2 membranes
Cis-snare pairing
if same membrane
Homotypic/ heterotypic fusion
fusing same or different things
v-SNARE and t-SNARE for exocytosis
vAMP and syntaxin
Mechanism of vesicle docking
- Rab GTP on vesicle binds to effector
- Assembly of snare complex -> V +T + Snap 25
- Membrane fusion
- cis-SNARE complex contacts a-SNAP on NSF protein
- SNARE complex disassembles using ATP
How are some ER proteins resident?
When a cop II vesicle buds, it selects proteins it needs but takes some liquid from compartment which contains soluble ER protein.
ER proteins have a KDEL sequence at C-terminal
-> enters golgi 1st compartment (cis-golgi)
-> pH in cis-golgi lower so KDEL receptor binds to protein
-> KDEL receptors part of COP1 protein coat
COP1 transfers vesicles from cis-golgi to ER
How do TM ER proteins stay resident?
When in cis-golgi lys-lys-x-x (KKXX) retrieval signal causes TM protein to be part of COP1 membrane
Secretion/ exocytosis from ER
A means of protein sorting
- Cop II transport vesicle from cis- medial to trans goglgi
- From trans either constitutitive secretion by vesicles to plasma membrane
- or regulated secretion into secretory vesicles by signal
e. g. insulin
As proteins move through golgi stack they’re modified, modifications include:
- early : phosphorylation of oligo sach. on lysosomal proteins
- cis-cist. -> removal of c. sugars
- medial -> +/-
After core glycosylation of a protein….
Trimming within the ER into golgi -> further adds proteins + sugars -> complex oligosach
Why is Endo H- sensitivity important?
Allows us to see at what point along the golgi a protein has reached. There’s a specific point X where proteins become endo-H resistant by golgi man II
O-linked glycosylation
- addition of sugars to OH serine/ theoirine by glycosyl transferase enzymes
How do proteins get from 1 golgi to another?
- Small vesicles bud off and fuse -> next -> balanced by backwards transport => net forward
- transport via cisternal maturation model
- > compartment will mature to next type of golgi balanced by vesticular transport through transport to protein compartment
- > acquires enzymes from next compartment while sending its own enzymes backwards
Lysosome
Degrades proteins + lipids
- > fuses with late endosome and reformed
- acidic pH
How proteins recruited get to lysosome?
- Mannose + lysosomal hydrolase precursor from ER in cis-golgi
- P-GlcNAc added to complex
- complex travels through golgi
- M6P signal uncovered at medial-golgi
- Binding to M6P receptor at trans golgi
- Clathrin coat forms and vesicle fuses with late endosome
- ATP hydrolysis energy used to remove P-GlcNAc
- Phosphate removed
- Late endosome becomes lysosome
If mannose 6-phosphate can’t be added…
lysosome hydroxylase secreted
- > mucolipidsis type II
- > non-functioning lysosome
Yeast screens
involved mutagenizing yeast and assaying for secretion of a vacuolar protease
